Atmospheric ice-nucleating particles in the eastern Mediterranean and the contribution of mineral and biological aerosol

While the atmosphere in the eastern Mediterranean is part of the dust belt, it encounters air masses from Europe, the Mediterranean Sea, and the Sahara and Arabian Desert that bring with them a whole host of potential dust and bioaerosol compositions and concentrations via long-range transport. The consequential changes in the populations of ice-nucleating particles (INPs), aerosols that influence weather and climate by the triggering of freezing in supercooled cloud water droplets, including in the convective cloud systems in the region, are not so well understood beyond the influence of desert dust storms in increasing INP concentrations. Here, we undertook an intensive INP measurement campaign in Israel to monitor changes in concentrations and activity from four major air masses, including the potential for activity from biological INPs. Our findings show that the INP activity in the region is likely dominated by the K-feldspar mineral content, with southwesterly air masses from the Sahara and easterly air masses from the Arabian Desert markedly increasing both aerosol and INP concentrations. Most intriguingly, a handful of air masses that passed over the Nile Delta and the northern Fertile Crescent, regions containing fertile agricultural soils and wetlands, brought high INP concentrations with strong indicators of biological activity. These results suggest that the Fertile Crescent could be a sporadic source of high-temperature biological ice-nucleating activity across the region that could periodically dominate the otherwise K-feldspar-controlled INP environment. We propose that these findings warrant further exploration in future studies in the region, which may be particularly pertinent given the ongoing desertification of the Fertile Crescent that could reveal further sources of dust and fertile soil-based INPs in the eastern Mediterranean region.